Effect of chemical crosslinking on mechanical and electrical properties of ultrahigh-molecular-weight polyethylene-carbon fiber blends prepared by gelation/crystallization from solutions

In an attempt to improve the mechanical property of polyethylene composite at high temperature, crosslinking of ultrahigh-molecular-weight polyethylene (UHMWPE) and carbon fiber (CF) blends was carried out by using dicumyl peroxide (DCP). The specimens were prepared by gelation/crystallization from solutions. The effect of chemical crosslinking on mechanical and electrical properties of UHMWPE/CF blends with composition of 1/0, 1/0.25, and 1/1 (w/w) were investigated in detail. Electrical conductivity and thermal mechanical properties of the blends with the 1/1 composition were greatly improved by incorporation of enough content of CF and adequate crosslinking network formation. Surprisingly, the Young’s modulus of the 1/1 blend reached 20 GPa at room temperature (20 °C). On the other hand, heat treatment at 135 °C played an important role for obtaining a high PTC effect for the UHMWPE-CF blend in which the PTC intensity reached 10⁷.     

Surface studies of ultra‐high molecular weight polyethylene irradiated with high‐energy pulsed electron beams in air

Ultra‐high molecular weight polyethylene (UHMWPE) was irradiated in air with high‐energy (9 MeV), pulsed electron beams to doses ranging from 2.5 to 100 Mrad and subsequently heat treated at 120°C for a time period of 120 min. Surface characterization of the target side of irradiated UHMWPE samples was carried out both before and after the heat treatment by means of attenuated total reflection Fourier‐transform infrared (FTIR/ATR) spectroscopy and microhardness measurement. The obtained results provided further evidence supporting our earlier observation (Tretinnikov, O. N.; Ogata, S.; Ikada, Y. Polymer 1998, 39, 6115) that thermal decomposition of hydroperoxides formed upon irradiation of UHMWPE with high‐energy, pulsed electron beams in air leads to surface crosslinking, and the subsequent surface hardening of the irradiated polymer. Importantly, we found that this phenomenon has the highest contribution to the surface hardness enhancement of the polymer when the radiation dose is in the range of 10–30 Mrad. In addition, we found that this irradiation and subsequent heat treatment of UHMWPE in air does not lead to formation of carbonyl‐containing products unless the radiation dose exceeds 20 Mrad. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1503–1512, 1999     

Study of the morphology,thermal and mechanical properties of irradiated isotactic polypropylene films

Thin films of isotactic polypropylene (iPP) are of great economical importance and their production is quite challenging due to the need of very fast uniaxial or biaxial expansion. During the expansion, critical problems usually arise, like structure disruption, shear thinning, causing material, energy and time losses. This work aims to study the surface morphology and compare the thermal, mechanical properties of PP films irradiated by gamma ray in an acetylene atmosphere after uniaxial expansion. PP films were made by compression molding at 190 °C with cooling in water at room temperature and irradiated by gamma ray, at (5, 12.5 and 20 kGy) under acetylene atmosphere. After irradiation the samples were submitted to thermal treatment at 90 °C for 1 h and then stretched out at 170 °C using an Instron machine. The surface of PP films, pristine and modified, (i.e., irradiated), was studied using optical microscopy (OM) and scanning electron microscopy (SEM). The changes in morphology, crystallinity and tensile parameters, like yield stress, rupture stress and elongation strain of the PP with irradiation dose were investigated. The results showed some evidences of gel formation due to crosslinking and/or long chain branching induced by radiation.     

Polypropylene nanogel: “Myth or reality”

The objective of this work is the investigation of the nanogel and microgel formation in modified PP. The modified PP in pellets was synthesized by gamma irradiation of pristine PP under a crosslinking atmosphere of acetylene in dose of 5, 12.5 and 20 kGy, followed by thermal treatment for radical recombination and annihilation of the remaining radicals. The thin film gel of the polypropylenes was obtained by extraction in boiling xylene for period of 12 h at 138 °C, followed by decantation in beaker at room temperature of 25 °C with the total volatilization of the xylene and deposition of dried material film on glass substrate under agitation by Settling process. The thin film gel formed of pristine PP and modified PP (i.e., irradiated) was characterized using scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM) and differential scanning calorimetry (DSC). The PP morphology indicated the nanogels and microgel formation with increase of spherulitic concentration and crystallinity at dose of 12.5 kGy.     

Effects of PVA,agar contents,and irradiation doses on properties of PVA/ws-chitosan/glycerol hydrogels made by γ-irradiation followed by freeze-thawing

Poly(vinyl alcohol) (PVA)/water soluble chitosan (ws-chitosan)/glycerol hydrogels were prepared by γ-irradiation and γ-irradiation followed by freeze-thawing, respectively. The effects of irradiation dose and the contents of PVA and agar on the swelling, rheological, and thermal properties of these hydrogels were investigated. The swelling capacity decreases while the mechanical strength increases with increasing PVA or agar content. Increasing the irradiation dose leads to an increase in chemical crosslinking density but a decrease in physical crosslinking density. Hydrogels made by irradiation followed by freeze-thawing own smaller swelling capacity but larger mechanical strength than those made by pure irradiation. The storage modulus of the former hydrogels decreases above 50 °C and above 70 °C it comes to the same value as that prepared by irradiation. The ordered association of PVA is influenced by both chemical and physical crosslinkings and by the presence of ws-chitosan and glycerol. These hydrogels are high sensitive to pH and ionic strength, indicating that they may be useful in stimuli-responsive drug release system.     

Effects of ultrahigh molecular weight polyethylene and mould temperature on morphological evolution of isotactic polypropylene at micro-injection moulding condition

The effects of ultrahigh molecular weight polyethylene (UHMWPE) and mould temperature (T_mould) on an isotactic polypropylene (iPP) matrix moulded via micro-injection were investigated via polarized light microscopy, scanning electron microscopy, differential scanning calorimetry, wide-angle X-ray diffraction and small-angle X-ray scattering. Results showed that the complex viscosity of the system increased significantly when the UHMWPE content was more than 5%; however, this viscosity decreased when the UHMWPE content was less than 5%. In addition, the addition of UHMWPE increased the onset of crystallisation temperature and the relative crystallinity of the β-form crystals in micro-injection moulded specimens. Moreover, the UHMWPE phase induced the formation of fan-shaped β crystals in iPP/UHMWPE blends. When mould temperature was 50 °C, the degree of orientation of microparts increased and the crystalline structures were highly compact. However, the relative crystallinity of the β-phase form (K_β) was lower than those prepared at 130 °C T_mould. Most importantly, well-oriented, bundle-like β crystals have been discovered for the first time in 5 wt.% UHMWPE/iPP blends obtained at 130 °C T_mould owing to the “orientation-maintenance” and “shear-amplification” effects of UHMWPE.     

Mechanical and tribological performance of UHMWPE influenced by temperature change

Water-lubricated surface bearing components experience boundary and mixed lubrication during operation. The lack of lubrication induces temperature increase, affecting the properties of the component. Ultra-high molecular weight polyethylene (UHMWPE) is commonly used for these applications and the influence of the temperature on the mechanical and tribological performance has not been clearly identified. This study evaluates the wear resistance and hardness of UHMWPE with the temperature increase in a range of 20 °C–60 °C. An important reduction of hardness and wear resistance was observed in this interval. The wear rate increased 94.8% when the temperature changed from 20 °C to 50 °C. The wear resistance decreased more rapidly than the hardness when the temperature was increased. The correlation between hardness and wear rate is less consistent when the hardness value was below 4.12 (Hv_0.3), reported at 40 °C. Plastic deformation and adhesion were highly enhanced with temperature.     

Preparation of aqueous crosslinked dispersions of functionalized poly(d,l-lactic acid) with a thermomechanical method

Aqueous crosslinked microparticle dispersions were prepared from a copolymer of d,l-lactic acid, 1,4-butanediol, and itaconic acid with a thermomechanical method. The copolymer was prepared in one step polycondensation reaction using Sn(Oct)₂ as a catalyst. A polymer with M_n of 2800 g mol^?1 and a molecular weight distribution of 1.41 was obtained (as determined by SEC), that contained double bonds introduced by the itaconic acid monomer units (6 mol-%, as determined by NMR). Crosslinking ability of the prepared copolymer was demonstrated in bulk by adding a thermal initiator and altering amounts of ethylene glycol dimethacrylate (EGDMA) crosslinking agent into molten polymer at 60–150 °C. A crosslinked gel was formed in less than 15 min at 80 °C when 10 wt.% of EGDMA was added and benzoyl peroxide (BPO) was used as the initiator. Aqueous dispersions were prepared of the non-crosslinked copolymer with a thermomechanical method that involved slow addition of aqueous polyvinyl alcohol (PVA) solution into molten copolymer at 60 °C under shear. Dispersions were prepared with 10 wt.% of EGDMA and 2 wt.% of BPO. Crosslinking of the dispersed microparticles was achieved by heating the dispersions at 80 °C for 30 or 60 min. The dispersions were characterized by SEM, DSC, TGA, FT-IR, solid state NMR, and gel content measurements. The effect of crosslinking was clearly seen in SEM images of films cast from the dispersions. The films cast from non-crosslinked dispersions had smooth morphology whereas in films cast from crosslinked dispersions separate spherical particles were observed. During the crosslinking reactions, glass transition temperatures increased (as determined by DSC), thermal stability of the samples increased (as determined by TGA), and the gel content of the samples increased.     

Fast responsive poly(<Emphasis Type="Italic">N,N</Emphasis>-diethylacrylamide) hydrogels with interconnected microspheres and bi-continuous structures

We prepared thermo-responsive polymer hydrogels by γ-ray irradiation of aqueous solutions of N, N-diethylacrylamide at different temperatures below and above its lower critical solution temperature (LCST). Poly(N, N-diethylacrylamide) gel had a transparent and homogeneous structure when the radiation-induced polymerization and crosslinking were carried out below the LCST (25 °C) of the polymer. On the other hand, cloudy and heterogeneous gels were formed at temperatures above the LCST of the polymer (>35 °C). From environmental scanning electron microscopy observations, the gels prepared at 35 and 40 °C were seen to show sponge-like bi-continuous porous structures, while those prepared at 50 °C showed a porous structure consisting of interconnected microspheres. For temperature changes between 10 and 40 °C, gels with porous structures showed rapid volume transitions on a time scale of about a minute, not only for shrinking but also for swelling processes, which is in remarkable contrast to the porous poly(N-isopropylacrylamide) hydrogels.     

Influence of α-Tocopherol Load and Annealing Treatment on the Wear Resistance of Biomedical UHMWPE Irradiated with Electron Beam

A biomedical UHMWPE electron beam irradiated in air with doses raging between 25 and 100 KGy was doped with different amounts of α-tocopherol (0.1–0.7 wt.%) to prevent oxidative degradation. The polymer was annealed just after the irradiation at 110° for 24 h. Physical and mechanical tests were performed on the UHMWPE before and after the irradiation and on neat or doped material. Results showed that heat treatment after irradiation improves cross-links in the UHMWPE and α-tocopherol has good antioxidant action. The UHMWPE blended with 0.7% of antioxidant became harder and more wear resistant (+52%) after the irradiation with 100 KGy.     

Crsslinking effect of ultrahigh molecular weight polyethylene-low molecular weight polyethylene blend films produced by gelation/crystallization from solutions

Polyethylene-polyethylene blend films were prepared by gelation/crystallization from semidilute solution by using ultrahigh molecular-weight (mw) polyethylene (UHMWPE) (mw=6×10⁶) and low molecular weight polyethylene (LMWPE) (mw=4×10⁴). The UHMWPE/LMWPE compositions chosen were 50/50, 67/33, and 91/9. Elongation was carried out in a hot oven at 115–130°C. The drawn films were exposed to an electron beam under nitrogen flow. Radiation doses chosen were 10, 20, 40, and 100 Mrad. crosslinking caused a significant effect in improving high temperature resistance for the blend film with draw ratio of 20 in the case of irradiation doses less than 20 Mrad. The elongation beyond 20 times and high doses beyond 20 Mrad hampered the crosslinking effect and the specimens were easily torn manually. This is thought to be due to the fact that the excess irradiation dose causes main chain scission apart from crosslinking.     

Dynamic and controlled rate thermal analysis of halotrichite

Three halotrichites namely halotrichite Fe²⁺SO₄·Al₂(SO₄)₃·22H₂O, apjohnite Mn²⁺SO₄·Al₂(SO₄)₃·22H₂O and dietrichite ZnSO₄·Al₂(SO₄)₃·22H₂O, were analysed by both dynamic, controlled rate thermogravimetric and differential thermogravimetric analysis. Because of the time limitation in the controlled rate experiment of 900 min, two experiments were undertaken (a) from ambient to 430 °C and (b) from 430 to 980 °C. For halotrichite in the dynamic experiment mass losses due to dehydration were observed at 80, 102, 319 and 343 °C. Three higher temperature mass losses occurred at 621, 750 and 805 °C. In the controlled rate thermal analysis experiment two isothermal dehydration steps are observed at 82 and 97 °C followed by a non-isothermal dehydration step at 328 °C. For apjohnite in the dynamic experiment mass losses due to dehydration were observed at 99, 116, 256, 271 and 304 °C. Two higher temperature mass losses occurred at 781 and 922 °C. In the controlled rate thermal analysis experiment three isothermal dehydration steps are observed at 57, 77 and 183 °C followed by a non-isothermal dehydration step at 294 °C. For dietrichite in the dynamic experiment mass losses due to dehydration were observed at 115, 173, 251, 276 and 342 °C. One higher temperature mass loss occurred at 746 °C. In the controlled rate thermal analysis experiment two isothermal dehydration steps are observed at 78 and 102 °C followed by three non-isothermal dehydration steps at 228, 243 and 323 °C. In the CRTA experiment a long isothermal step at 636 °C attributed to de-sulphation is observed.     

Crosslinking of polyimides via spirodilactone unit in polymer backbone

A new approach for the crosslinking of polyimides via the lactamization of spirodilactone unit in polyimide backbone was studied by two means: model reaction and the comparison of the properties of the polyimide precursors to those of the crosslinking polymers. Polyimides 4 and 5 were soluble in N,N′dimethylacetamide (DMAc), dimethylformamide (DMF), dimethylsulfoxide (DMSO), N′-methylpyrrolidone (NMP), and other common organic solvents, whereas their corresponding crosslinking polymers were insoluble in these solvents. The glass transition temperatures for polyimide 5 and its crosslinking polymer were 262°C and 291°C, whereas those for polyimide 4 and its crosslinking polymer were 265°C and 360°C. The weight-loss rate of the crosslinking polymers was apparently slower than that of the precursors when the temperature was > 400°C. The 10% weight-loss temperature for the polyimides 4 and 5 was < 500°C, whereas that for the crosslinking polymers was close to or above 600°C. The results indicate that this type of crosslinking polymer has good thermal properties. The temperature for the formation of lactam was above 180°C. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3680–3686, 1999     

Comparison between thermal crosslinkers based on melamine-formaldehyde and benzoguanamine resin and their influence on main performance of acrylic pressure-sensitive adhesives as tack,peel adhesion,shear strength and pot-life

An acrylic pressure-sensitive adhesive (PSA) was synthesized in ethyl acetate at about 80 °C by the use of 2-ethylhexyl acrylate, ethyl acrylate, methyl acrylate and acrylic acid at presence of thermal radical initiator AIBN. The synthesized acrylic PSA was crosslinked at relatively low temperatures at about 110–125 °C using thermal crosslinkers selected from melamine-formaldehyde resins and benzoguanamine resins. The crosslinking process runs between carboxylic groups of acrylic PSA and reactive groups from investigated amine resins. The choice of suitable thermal reactive crosslinkers has significant and relevant influence at presence of organic acid catalysts on main performance of crosslinked PSA such as tack, peel adhesion and shear strength.     

Synthesis and radical crosslinking reaction of poly (vinylcyclopropanone acetal)s

Synthesis and radical crosslinking reaction of poly (vinylcyclopropanone acetal)s ( 4 ), volume change on the crosslinking, and thermal analysis of 4 and the crosslinked polymers were carried out. 4 was prepared by the reaction of 1,1-dichloro-2-vinylcyclopropane with sodium dialkoxides. Radical crosslinking of 4a was carried out in the presence of AIBN, BPO, or DTBP at 60, 80, or 120°C, respectively. 4a afforded no dichloromethane-insoluble part in the crosslinking in chlorobenzene but afforded in 37% yield in bulk at 120°C. On the other hand, 4b afforded dichloromethane-insoluble part in 33% yield in the crosslinking in chlorobenzene at 120°C. Volume shrinkage of 4a and 4b on crosslinking at 120°C was 4.11 and 3.55%, respectively. Glass transition temperatures of the crosslinked polymers obtained from 4a and 4b were 28 and 163°C, respectively, which were 32 ～ 49°C higher than those before crosslinking. © 1994 John Wiley & Sons, Inc.     

Effects of annealing and the addition of PEG on the PVA based hydrogel by gamma ray

Poly(vinyl alcohol) (PVA) is an interesting material with good biocompatibility, high elasticity and hydrophilic characteristics. PVA hydrogels have been formed through chemical crosslinking with aldehyde, photopolymerization and physical crosslinking with freeze-thawing. In this study, crosslinked hydrogels based on PVA, and poly(ethylene glycol) (PEG) were prepared by gamma-ray irradiation, and then annealed at 120 °C. The properties of a hydrogel such as gel fraction, swelling behavior, gel strength as a function of PEG content and annealing time were investigated. Also, the thermal behaviors were examined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The gel fraction decreases with an increase in PEG content and decrease in annealing time. The tensile strength increases with an increase in annealing time. The thermal behaviors have shown different patterns according to the annealing time. The improved properties suggest that PVA/PEG blend hydrogel can be a good candidate for applications in the articular cartilage.     

The Influence of SBF on Surface Properties of Irradiated GO/UHMWPE Nanocomposites

In this study, the irradiated ultra-high molecular weight polyethylene (UHMWPE) components doping with graphene oxide (GO) were immersed in simulated body fluid (SBF) environment at 37°C for 6 months, and their surface properties were studied by ball indentation, scratch, and wetting tests. The results show that both the irradiation cross-linking treatment and the addition of 0.5 wt % GO can increase the indentation hardness and scratching coefficient of the surface of UHMWPE composites, and wettability of it becomes better. After soaking in SBF, the ball indentation and scratch coefficient of irradiated GO/UHMWPE nanocomposites decreased by 12.4 and 10.0%, respectively. Which may due to a swelling action and the oxidative degradation occurred on the surface of GO/UHMWPE nanocomposites. However, irradiation cross-linking and filling with GO can prevent the macromolecules from penetrating into UHMWPE base material and materials, thereby reducing the rate of swelling and oxidative degradation.     

Synthesis of high specific surface area YSZ (ZrO<Subscript>2</Subscript>–8Y<Subscript>2</Subscript>O<Subscript>3</Subscript>) nanocrystalline powder by modified polymerized complex method

In this study high specific surface area yttria-stabilized zirconia (ZrO₂–8Y₂O₃) nanocrystalline powder have been synthesized through “modified polymerized complex (MPC) method”. Zirconium chloride, yttrium nitrate, citric acid and ethylene glycol were polymerized at 80 °C to produce a gel-like mass in which metallic ions were uniformly distributed. During the thermal treatment of dried gel, nanocrystalline YSZ powder was formed. Thermal reactions and phase formation of dried gel were investigated through thermal analysis (DTA/TG) and X-ray diffraction (XRD) analysis, respectively. Chemical bonding and thermal decomposition behavior of dried gel was investigated by FTIR analysis. During decomposition, the nature of the bonding between carboxylate groups and the cations changed from unidentate to bridging at 370 °C and carbonate species were detected at 470 °C. Morphology of powder calcined at 650 °C was analyzed by scanning electron microscope (SEM). YSZ powder with high specific surface area was prepared successfully by this method.     

Thermal characterization of raw material pentoxifylline using thermoanalytical techniques and Pyr-GC/MS

This work studied the thermal characterization of the pentoxifylline raw material through thermoanalytical techniques (TG, DSC, DSC-photovisual) and analysis of degradation products by Pyr-GC/MS. The picture obtained with DSC-photovisual showed the total vaporization of pentoxifylline at 230.0 °C. The TG dynamical curve presented only one step for the loss of mass evidencing to be a kinetic process of zero order reaction. The pyrograms obtained for pentoxifylline sample in the solid state and solution in the temperatures of 250.0, 300.0, and 400.0 °C, showed only one peak identifying the pentoxifylline.     

Photo-crosslinked acrylates degradation kinetics

A series of crosslinked polyurethane acrylate solids with glass transition temperatures ranging from –49 to +65 °C was prepared by photopolymerization of specially formulated solvent-free resins. The kinetics of thermooxidative and thermal (in N₂) degradation of these crosslinked acrylate networks at temperatures ranging from 100 to 400 °C was studied as a function of crosslink density using thermogravimetry. The polyacrylate network degradation rate decreased with the increase of crosslink density, while apparent activation energy of degradation increased. Polyacrylate thermal stability increase with crosslinking was explained by decreased rate of oxygen and volatile products diffusion and/or slowing of depolymerization due to increased radical recombination rate, and decreased chain segments mobility in systems with higher crosslink density.